The present invention relates to improvement of a fuel cell system that performs low-efficiency operation for a warm-up during a startup.
When a fuel cell starts operating in a low-temperature environment at or lower than the freezing point, the water remaining in a cathode electrode, an anode electrode, or a diffusion layer freezes, hindering the gas flow, and the water remaining in the polymer electrolyte membrane freezes, thereby decreasing the proton conductivity. For this reason, in a low-temperature startup, a warm-up operation of fuel cell is performed and then a transition is made to a high-efficiency operation.
In the warm-up operation of the fuel cell, the power generation efficiency is decreased and the amount of self-generated heat is increased to enhance the warm-up. For example, Japanese Patent Application Laid-open No. 2005-174645 (paragraphs 0013, 0037 to 0048) describes a method of performing the warm-up operation of a fuel cell in a state in which the flow rate of the oxidizing agent gas supplied to part of the fuel cell is reduced or terminated. With such a method, a reduction reaction proceeds in the portion where the oxidizing agent gas is absent, hydrogen is generated, and heat is produced, thereby ensuring intensive increase in temperature.
National Publication of the translation of International Application No. 2003-504807 (paragraph 0009) describes a method by which a deficit of reaction product is similarly induced in part of a fuel cell, the degree of overvoltage in this part is increased, and heat is generated.
However, in the conventional fuel cell system that performs low-efficiency operation, the fuel cell temperature is monitored and the low-efficiency operation is stopped when the temperature rises (for example, Japanese Patent Application Laid-open No. 2005-174645, paragraph 0040), but when the amount of oxidizing agent gas supplied during the low-efficiency operation is too large, the exothermal reaction does not occur, an excess amount of water is produced, and there is a risk of inhibiting the warm-up operation.